1. Field of the Invention
The present invention relates to a method of manufacturing a polycrystalline silicon (poly-Si) film oriented along the crystal axis &lt;100&gt; by utilizing plasma.
2. Description of the Related Art
Vigorous researches and developments are being made nowadays with respect to the increase in the crystal grain size and orientation of the crystal axis of a poly-Si film used in a thin film transistor. Where a structure of metal electrode-gate insulator film (silicon oxide film or the like)-silicon is employed in a thin film transistor, the density of a surface states at the interface between the gate insulator film and the silicon substrate, which resides near the conduction and valence band edges in the energy band gaps of a semiconductor, is strongly dependent on the crystal axis direction perpendicular to the silicon surface. It is reported that, in the case of a single-crystalline silicon, the surface state density in the &lt;110&gt; direction is about three times as high as that in the &lt;100&gt; direction, and that the density in the &lt;111&gt; direction is about 6 to 7 times as high as that in the &lt;100&gt; direction. Under the circumstances, a single-crystalline silicon having a surface along the &lt;100&gt; crystal axis is used for the above-noted purpose.
In the case of using a poly-Si film, it is of high industrial importance to develop a silicon film having the crystal axis direction of the crystal grains oriented in the &lt;100&gt; direction relative to the surface thereof.
Also, an interface unavoidably resides between crystal grains in a poly-Si. As a result, a potential barrier, which is caused by the interfacial potential (i.e., charge level formation between energy band gaps of a semiconductor), is generated between crystal grains. In order to suppress the effect given by the interface between the crystal grains, it is necessary to increase the crystal grain size.
A poly-Si film having the crystal axis oriented in the &lt;100&gt; direction can be obtained by controlling the gas pressure and the depositing temperature in the step of thermally decomposing a feed gas containing a silane-series gas under a low gas pressure. Where the deposition is performed at about 600.degree. C. or lower in this method, however, the deposited silicon film certainly has a smooth surface, but is made amorphous. It is certainly possible to select conditions which permits deposition of a poly-Si film at about 600.degree. C. or more. In this case, however, the deposited poly-Si film has a very rough surface. It should be noted that the surface roughness is greatly dependent on the degree of orientation. If the degree of orientation is high, the surface roughness is increased to reach 0.2 .mu.m or more.
For preventing the surface roughness, it is known to the art to deposit first an amorphous silicon film by the pyrolytic deposition method at a depositing temperature of about 600.degree. C. or less, followed by annealing the amorphous silicon film at about 600.degree. C. or less for the solid phase crystallization. This method is certainly effective for obtaining a poly-Si film having a smooth surface and a large crystal grain size, but is incapable of controlling the crystal axis direction of the grown crystal grains. It follows that the crystal axis is positioned at random or is slightly oriented in the &lt;111&gt; direction in many cases, resulting in failure to manufacture a poly-Si film oriented in &lt;100&gt; direction.